Implement fallback for V2 invite API

If the room version is either 1 or 2 then a server should retry failed
`/v2/invite` requests with the v1 API

synapse/federation/federation_client.py

@@ -751,18 +751,9 @@ def send_request(destination):
 
     @defer.inlineCallbacks
     def send_invite(self, destination, room_id, event_id, pdu):
-        time_now = self._clock.time_msec()
-        try:
-            code, content = yield self.transport_layer.send_invite(
-                destination=destination,
-                room_id=room_id,
-                event_id=event_id,
-                content=pdu.get_pdu_json(time_now),
-            )
-        except HttpResponseException as e:
-            if e.code == 403:
-                raise e.to_synapse_error()
-            raise
+        room_version = yield self.store.get_room_version(room_id)
+
+        content = yield self._do_send_invite(destination, pdu, room_version)
 
         pdu_dict = content["event"]
 
@@ -780,6 +771,55 @@ def send_invite(self, destination, room_id, event_id, pdu):
 
         defer.returnValue(pdu)
 
+    @defer.inlineCallbacks
+    def _do_send_invite(self, destination, pdu, room_version):
+        """Actually sends the invite, first trying v2 API and falling back to
+        v1 API if necessary.
+
+        Args:
+            destination (str): Target server
+            pdu (FrozenEvent)
+            room_version (str)
+
+        Returns:
+            dict: The event as a dict as returned by the remote server
+        """
+        time_now = self._clock.time_msec()
+
+        try:
+            content = yield self.transport_layer.send_invite_v2(
+                destination=destination,
+                room_id=pdu.room_id,
+                event_id=pdu.event_id,
+                content={
+                    "event": pdu.get_pdu_json(time_now),
+                    "room_version": room_version,
+                    "invite_room_state": pdu.unsigned.get("invite_room_state", []),
+                },
+            )
+            defer.returnValue(content)
+        except HttpResponseException as e:
+            if e.code in [400, 404]:
+                if room_version in (RoomVersions.V1, RoomVersions.V2):
+                    pass  # We'll fall through
+                else:
+                    raise Exception("Remote server is too old")
+            elif e.code == 403:
+                raise e.to_synapse_error()
+            else:
+                raise
+
+        # Didn't work, try v1 API.
+        # Note the v1 API returns a tuple of `(200, content)`
+
+        _, content = yield self.transport_layer.send_invite_v1(
+            destination=destination,
+            room_id=pdu.room_id,
+            event_id=pdu.event_id,
+            content=pdu.get_pdu_json(time_now),
+        )
+        defer.returnValue(content)
+
     def send_leave(self, destinations, pdu):
         """Sends a leave event to one of a list of homeservers.
 

synapse/federation/transport/client.py

@@ -21,7 +21,7 @@
 from twisted.internet import defer
 
 from synapse.api.constants import Membership
-from synapse.api.urls import FEDERATION_V1_PREFIX
+from synapse.api.urls import FEDERATION_V1_PREFIX, FEDERATION_V2_PREFIX
 from synapse.util.logutils import log_function
 
 logger = logging.getLogger(__name__)
@@ -289,7 +289,7 @@ def send_leave(self, destination, room_id, event_id, content):
 
     @defer.inlineCallbacks
     @log_function
-    def send_invite(self, destination, room_id, event_id, content):
+    def send_invite_v1(self, destination, room_id, event_id, content):
         path = _create_v1_path("/invite/%s/%s", room_id, event_id)
 
         response = yield self.client.put_json(
@@ -301,6 +301,20 @@ def send_invite(self, destination, room_id, event_id, content):
 
         defer.returnValue(response)
 
+    @defer.inlineCallbacks
+    @log_function
+    def send_invite_v2(self, destination, room_id, event_id, content):
+        path = _create_v2_path("/invite/%s/%s", room_id, event_id)
+
+        response = yield self.client.put_json(
+            destination=destination,
+            path=path,
+            data=content,
+            ignore_backoff=True,
+        )
+
+        defer.returnValue(response)
+
     @defer.inlineCallbacks
     @log_function
     def get_public_rooms(self, remote_server, limit, since_token,
@@ -958,3 +972,24 @@ def _create_v1_path(path, *args):
         FEDERATION_V1_PREFIX
         + path % tuple(urllib.parse.quote(arg, "") for arg in args)
     )
+
+
+def _create_v2_path(path, *args):
+    """Creates a path against V2 federation API from the path template and
+    args. Ensures that all args are url encoded.
+
+    Example:
+
+        _create_v2_path("/event/%s/", event_id)
+
+    Args:
+        path (str): String template for the path
+        args: ([str]): Args to insert into path. Each arg will be url encoded
+
+    Returns:
+        str
+    """
+    return (
+        FEDERATION_V2_PREFIX
+        + path % tuple(urllib.parse.quote(arg, "") for arg in args)
+    )